The present invention is directed to audio speakers, sometimes referred to as loudspeakers, and especially to reducing distortion caused by non-linear characteristics in audio speakers.
In recent years, loudspeaker engineers have begun employing various servo-related technologies in the design of loudspeakers seeking to reduce distortion and modify the dynamics of the speaker and its enclosure. For example, in a subwoofer, cone excursions can be quite large, especially at low frequencies, leading to suspension non-linearities that result in significant distortion. Motional feedback signals combined with carefully designed compensators can alleviate these distortion problems. In addition, motional feedback signals can be employed to modify the suspension properties allowing designers to modify the speaker's response without having to physically modify the enclosure or the speaker design. Important impediments to widespread adoption of such technologies have been the costs associated with implanting sensors in the diaphragm of the speaker to measure or monitor cone motion and the size and mass of the sensors. The costs reduced profit margins sufficiently to make the improvements unattractive. The size has been a design challenge for small, compact speaker units of the sort often sought in today's market. If the mass of a sensor is too great it will interfere with or skew the performance of a speaker.
U.S. Pat. No. 3,047,661 to Winker for “High Fidelity Audio System”, issued Jul. 31, 1962, discloses an arm in contact with a speaker cone for operating a sensor. The arm responds to motion by the speaker cone to actuate any of a variety of transducers: capacitive (Winker; FIGS. 1 and 2), ionization chamber (Winker; FIG. 3) and resistance bridge (Winker; FIG. 4). It is important that the indication of speaker cone movement be as directly associated with the movement as possible and interfere with the movement as little as possible. The mass of the sensor in contact with the speaker should preferably be small as compared to the mass of the speaker cone. It would be advantageous to avoid moving the masses associated with actuating Winker's various disclosed embodiments of transducers to reduce the affect the sensor arm has upon motion of the speaker cone and to more directly indicate that movement.
Another approach to sensing movement of a speaker cone is disclosed in U.S. Pat. No. 4,727,584 to Hall for “Loudspeaker with Motional Feedback”, issued Feb. 23, 1988. Hall discloses mounting an accelerometer on a loudspeaker coil. However, such an arrangement requires providing electrical leads to the accelerometer. Hall's apparatus adds mass and bulk that can skew indications of cone motion, risk wire breakage from metal fatigue associated with motion of the cone and limit how compactly the speaker may be made. Other aspects of Hall's apparatus, such as a requirement for a dust cap, add further to the cost and bulk to a speaker.
U.S. Pat. No. 3,821,473 to Mullins for “Sound Reproduction System with Driven and Undriven Speakers and Motional Feedback”, issued Jun. 28, 1974, discloses using other types of sensors mounted within the speaker cone on the face of the driving transducer. Mullins discloses using a variety of sensing technologies for his sensors, including “piezoelectric, piezoresistive, strain gauges, pressure sensitive paint, mass balance or any other transducer which will produce an output that is proportional to acceleration” [Mullins; Col. 4, lines 54-57].
Others have attempted to provide indication of speaker cone motion using a variety of electromagnetic coil structures coaxially arranged with the speaker voice coil. Such apparatuses add complexity, cost and bulk to a speaker. Examples of such coaxially arranged electromagnetic coil structures are U.S. Pat. No. 4,243,839 to Takahashi et al. for “Transducer with Flux Sensing Coils”, issued Jan. 6, 1981; U.S. Pat. No. 4,550,430 to Meyers for “Sound Reproducing System Utilizing Motional Feedback and an Improved Integrated Magnetic Structure”, issued Oct. 29, 1985; U.S. Pat. No. 4,573,189 to Hall for “Loudspeaker with High Frequency Motional Feedback”, issued Feb. 25, 1986; U.S. Pat. No. 4,609,784 to Miller for “Loudspeaker with Motional Feedback”, issued Sep. 2, 1986; and U.S. Pat. No. 5,197,104 to Padi for “Electrodynamic Loudspeaker with Electromagnetic Impedance Sensor Coil”, issued Mar. 23, 1993.
Another approach to sensing motion of speaker cones has been to use Hall Effect sensors, as disclosed in U.S. Pat. No. 4,821,328 to Drozdowski for “Sound Reproducing System with Hall Effect Motional Feedback”, issued Apr. 11, 1989. Drozdowski's apparatus requires including a Hall Effect sensor within the cone and providing electrical leads for communicating with the sensor from outside the cone. It is a complex arrangement fraught with opportunities for breakdown and adds cost, bulk and mass to a speaker.
Yet another approach to monitoring speaker cone motion has involved the use of optical sensor technology, as disclosed in U.S. Pat. No. 4,207,430 to Harada et al. for “Optical Motional Feedback”, issued Jun. 10, 1980. A significant problem with using optical sensor systems in addition to adding complexity, cost, mass and bulk is that they are subject to being rendered less efficient, unreliable or even inoperative by dust or other debris buildup.
There is a need for an inexpensive, low mass and compact apparatus and method for monitoring or measuring speaker cone displacement in audio speakers that does not significantly affect operation of a speaker.